Environmental control system and method

ABSTRACT

The present invention provides a sequencing control strategy for environmental system controllers that takes better advantage of the capabilities of the system elements to enhance performance and reduce operating costs. Digital controller technology is included and operates in accordance with a state transition diagram clarifying conditions that must exist for the environmental controller to switch from one mode of operation to another (e.g., from a cooling mode with dampers set at the minimum position to a cooling mode with the dampers modulating to reduce the energy used for mechanical cooling). Several controllers, or a single controller operating on several sets of control parameters, sequentially operate in accordance with transition data and system performance characteristics for controlling system operation.

FIELD OF THE INVENTION

This invention relates generally to environmental control systems forheating, ventilating and air conditioning (HVAC) applications and moreparticularly, to a system and method of controlling elements of anenvironmental control system.

BACKGROUND OF THE INVENTION

Environmental control systems such as heating, ventilating and airconditioning (HVAC) systems are well known and are designed andimplemented to maintain environmental conditions within buildings. Atypical installation sees the building divided into zones, and the HVACsystem is adapted to maintain each of the zones within predefinedenvironmental parameters (e.g., temperature, humidity,outdoor-recirculated air ratio, etc.). In this exemplary installation,an air distribution system connects each of the zones with an airhandling unit (AHU) for providing a supply of conditioned air to thezones.

The AHU generally includes elements for introducing outdoor air into thesystem and for exhausting air from the system; elements for heating,cooling, filtering and otherwise conditioning the air in the system; andelements for circulating the air within the air distribution system at adesired flow rate. The AHU also includes a controller to control theoperation of these elements. A primary task of the AHU is to providesupply air to each of the particular zones to offset the thermal loadsimposed on the zone to maintain a comfortable environment for theoccupants. Because thermal loads for the zones can vary markedly, it iscommon for an AHU to be controlled to maintain the supply airtemperature at a setpoint value that is sufficiently low to satisfy thezone with the largest load at any given time. If needed, the air streamis throttled and/or reheated at terminal boxes to provide adequatecomfort in all zones.

AHU controllers commonly use sequencing logic to determine the mosteconomic way to utilize the elements of the AHU to minimize the cost ofmaintaining the supply air temperature at the setpoint value. Forinstance, it is not uncommon for a building subjected to large dailytemperature swings to require mechanical heating in the morning andmechanical cooling in the afternoon. Costs associated with mechanicalcooling can be reduced using economizer cycle control. Economizer cyclecontrol involves modulating dampers located in the mixing box to controlthe amount of outdoor air that is introduced to the AHU. Under theproper outdoor air conditions and economizer cycle control, the supplyair temperature can be maintained at the setpoint value without the use(or with reduced use) of mechanical cooling.

Economizer cycle control logic typically involves a comparison of theoutdoor and return air temperatures or enthalpies. If the outdoor airtemperature is greater than some minimum value and less than the returnair temperature, an opportunity exists to reduce mechanical coolingcosts.

On the surface, sequencing logic and economizer control are quiteintuitive. In practice, however, the logic can be very difficult tofollow and just as challenging to implement because of numerousexceptional cases (e.g., freeze control, etc.) that must be addressed.This was especially true with pneumatic control systems. It has beendemonstrated for pneumatic control systems of actual AHUs that theoutdoor air damper and heating coil valve may cycle between fully openand fully closed approximately every two minutes. This wastes energy andleads to excessive component wear. The advent of digital control hasdone little to improve the situation because, rather than taking fulladvantage of the programming capabilities of digital controllers, logicused in pneumatic controllers has simply been adapted to the digitalcontrollers.

SUMMARY OF THE INVENTION

The present invention provides a sequencing control strategy forenvironmental system controllers that takes better advantage of thecapabilities of the system elements to enhance performance and reduceoperating costs. Digital controller technology is included and operatesin accordance with a state transition diagram clarifying conditions thatmust exist for the environmental controller to switch from one mode ofoperation to another (e.g., from a cooling mode with dampers set at theminimum position to a cooling mode with the dampers modulating to reducethe energy used for mechanical cooling). Several controllerssequentially operate in accordance with transition data (the statetransition diagram) and in response to system performancecharacteristics for controlling system operation.

In a preferred embodiment of the present invention, the environmentalcontrol system includes several controllers each optimized forcontrolling an associated one of the environmental system elements. Inaccordance with state transition logic retained in memory and accessedby the controllers, system control is passed between the controllersdepending on the required operating mode in view of measured systemperformance data.

In an alternate preferred embodiment of the present invention, theenvironmental control system includes a single controller adapted toaccess one of several sets of control parameters each of which isoptimized for controlling an associated one of the environmental controlsystem elements. In accordance with state transition logic, thecontroller operates a preferred set of control parameters depending onthe required operating mode in view of measured system performance data.

In still an additional preferred embodiment of the present invention auser interface including a representation of the state transition logicenhances system operation. The interface allows simplified viewing ofsystem operation and faults. In addition, the interface provides avehicle for simplified system modification.

These and other advantages and features of the present invention will bereadily appreciated by one of ordinary skill in the art from thefollowing detailed description of the preferred embodiments, thesubjoined claims and the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an air handling unit (AHU);

FIG. 2 is a block diagram illustrating a prior art AHU controller;

FIG. 3 is a block diagram illustrating an AHU controller in accordancewith a preferred embodiment of the present invention;

FIG. 4 is a block diagram illustrating an AHU controller in accordancewith an alternate preferred embodiment of the present invention;

FIG. 5 is a diagram illustrating the state transition data in accordancewith a preferred embodiment of the present invention;

FIG. 6 is a block diagram illustrating a variable-air-volume (VAV)device adapted in accordance with the present invention;

FIG. 7a is a diagram illustrating state transition data in accordancewith a preferred embodiment of the present invention for use with theVAV device of FIG. 6;

FIG. 7b is a diagram illustrating state transition data in accordancewith a preferred embodiment of the present invention for use with theVAV device of FIG. 6;

FIG. 8 is a representation of a graphic user interface in accordancewith a preferred embodiment of the present invention;

FIG. 9 is a further representation of a graphic user interface inaccordance with a preferred embodiment of the present invention;

FIG. 10 is a further representation of a graphic user interface inaccordance with a preferred embodiment of the present invention; and

FIG. 11 is a further representation of a graphic user interface inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in terms of several preferredembodiments. It should be understood from the outset that the presentinvention has a wide range of applications extending far beyond theexamples described herein. Referring then to FIG. 1 the components ofthe schematically illustrated AHU 10 used to maintain the supply airtemperature at the setpoint value are shown. This is done for simplicityand should in no way imply that the invention is limited to theseelements.

Air enters AHU 10 through the outdoor air damper 12 and, depending onthe mixing box 14 damper settings (controlling the positions of outdoorair damper 12, exhaust air damper 36 and recirculation damper 16 viaactuation of damper motors 38, 40 and 42, respectively), may be mixedwith air passing through the recirculation air damper 16. Air within AHU10 is circulated by supply fan 18 and return fan 20, respectively. Thetemperatures and flow rates of the outdoor and recirculation air streamswill determine the conditions of the supply air exiting mixing box 14.The return air temperature/humidity and flow are measured bytemperature/humidity sensor 24 and flow sensor 26. The outdoor airtemperature/humidity is measured by temperature/humidity sensor 28. Theair exiting mixed air plenum 14 passes through heating coil 30 andcooling coil 32. At most only one of the two coils will be active at anygiven time assuming the sequencing control strategy is implementedproperly and there are no valve leaks or other faults in the system.After being conditioned, the air is distributed to the zones through thesupply air ductwork 34. The supply air temperature is measureddownstream of the supply fan 18 by temperature/humidity sensor 36.Return air is drawn from the zones by the return fan 20 and is eitherexhausted or recirculated, depending once again on the position of themixing box 14 dampers.

The air handling unit controller 22 includes a controller (orcontrollers) to control the heating coil valve 44, the cooling coilvalve 46, and the damper motors 38-42, and control logic to determinethe component(s) (heating coil, cooling coil, dampers, cooling coil anddampers) to use to maintain the supply air temperature at the setpointvalue at any given time.

FIG. 2 shows a flow chart for a prior art sequencing control strategy200 which may be implemented in the air handling unit controller 22shown in FIG. 1. Control strategy 200 is based on strategies used inpneumatic control systems. A single feedback controller 202, usually aproportional-integral (P1) controller, is used with this strategy, inconjunction with economizer logic 204 and low select logic 206, toreduce component costs. The controller output is determined by comparingthe supply air temperature to a setpoint. If the scaled output fromfeedback controller 202 is between 100% and 200%, mechanical cooling viacooling coil 32 is used to cool the air. Here 100% represents nomechanical cooling and 200% represents maximum mechanical cooling. Ifthe outdoor air conditions are suitable, an economizer cycle 204(outdoor air dampers fully open) is used simultaneously to reduce themechanical cooling load. If the output from feedback controller 202 isbetween -100% and 0%, heating coil 30 is used to heat the supply air andthe outdoor air damper is at its minimum position determined byventilation criteria. If the output from the feedback controller isbetween 0% and 100%, outdoor air and return air are mixed in mixed airplenum 14 to produce supply air at the setpoint temperature. This isreferred to as free cooling because neither mechanical heating orcooling is used.

The dynamic characteristics of the three processes (i.e., heating,cooling, and free cooling) are significantly different, in which casethe use of a single feedback controller is limiting. To maintain stablecontrol, the controller must to be tuned for the worst case conditions.If this is the case, the closed loop response for other conditions willtend to be sluggish. If the feedback controller is not tuned for theworst case conditions, then valves 44 and 46 and dampers 12, 16 and 36may cycle between fully open and fully closed with resultant energywaste and component wear.

The control performance can be improved by using an adaptive controllersuch as disclosed and described in commonly assigned U.S. Pat. Nos.5,355,305, 5,506,768 and 5,568,377 the disclosures of which are herebyexpressly incorporated herein by reference, to adjust the proportionalgain and integral time of controller 202. However, the parameters mayneed significant adjustment as the control component changes, that is,as the control changes from cooling to heating. Also, it may bedifficult to tune at the transition region because the combined processmay be very nonlinear. During the time period that the adaptivecontroller is adjusting parameters at the transition region, the controlperformance may be sacrificed.

With reference to FIG. 3, in accordance with a preferred embodiment ofthe present invention, an air handling unit controller 22 providescontrol of the supply air temperature using three separate feedbackcontrollers 302, 304 and 306. Controller 302 is dedicated to controllingheating coil valve 44, controller 304 is dedicated to controllingcooling coil valve 46, and controller 306 is dedicated to controllingmixed air plenum 14 (i.e., damper motors 38-42 for controlling thepositions of dampers 12, 16 and 36, respectively) in accordance with aset of control parameters. These control parameters are optimized forthe particular control element, e.g., the parameters utilized bycontroller 302 are optimized for heating coil valve 44 control. At anygiven time, only one controller is operating. Each of controllers 302-306 are coupled to and access a memory in which transition data 308 isretained defining the transitions between operating modes, and hencewhich of the controllers are actively operating. Air handling unitcontroller 22 receives system performance data via inputs 310, and inaccordance with transition data 308 defining which controller isoperating, provides control outputs 312 for controlling the systemoperation.

With reference to FIG. 4, an alternative preferred embodiment of airhandling unit controller 22 sees the use of one controller 402 withthree sets of controller parameters 404 -408 corresponding to thevarious modes of operation retained in memory 410. Memory 410 alsoincludes transition data 412 for defining transitions between operatingmodes, and hence which set of controller parameters are executed bycontroller 402. Controller 22 receives system performance data viainputs 414, and in accordance with the transition data 412 definingwhich controller is operating, provides control outputs, 416 forcontrolling the system operation.

FIG. 5 shows a state transition diagram illustrating transition data308/412 and the controller parameters associated with the operatingmodes, respectively, in accordance with either preferred implementation.The states shown in FIG. 5 are described below.

State 1

In State I, feedback control is used to modulate the flow of hot waterto the heating coil 30 via valve 44, thereby controlling the amount ofenergy transferred to the air. Meanwhile, the mixing box dampers (12, 16and 36) are positioned for the minimum outdoor air required forventilation and the cooling coil valve 46 is closed. The transition toState 2 occurs after the control signal has been saturated at the noheating position for a time period equal to the state transition delay.For all state transitions, the current state must be continuouslysaturated at either its minimum or maximum limit for a period of timeequal to the state transition delay before operation can switch to a newstate.

State 2

In State 2, feedback control is used to adjust the position of dampers12, 16 and 36 in order to maintain the supply air temperature at thesetpoint value. Adjusting the positions of dampers 12, 16 and 36 variesthe relative proportion of outdoor air and return air in the supply airstream. In State 2, the heating and cooling coil valves 44 and 46 areclosed. Transition to State 1 occurs after the control signal for thedampers has been at the minimum outdoor air position for a time periodequal to the state transition delay. Transition to State 3 occurs afterthe control signal for the dampers has been at the maximum outdoor airposition for a time period equal to the state transition delay.

State 3

In State 3, feedback control is used to modulate the flow of cold waterto the cooling coil 32 via valve 46, thereby controlling the amount ofenergy extracted from the air. The outdoor air damper 12 is set atcompletely open and the heating coil valve 44 is closed. Transition toState 2 occurs after the control signal for mechanical cooling has beensaturated at the no cooling position for a time period equal to thestate transition delay. Economizer logic is used to determine thetransition to State 4. Either an enthalpy based or temperature basedeconomizer logic can be used. In the state diagram shown in FIG. 3,logic based on outdoor air temperature is used to determine thetransition point. Transition to State 4 occurs when the outdoor airtemperature is greater than the changeover temperature plus the deadbandtemperature. As an example, the changeover temperature could equal thereturn air temperature, and the deadband could equal 0.56° C. Thepurpose of the deadband temperature is to prevent cycling from State 3to State 4 due to noise in the return and/or outdoor air temperaturesensors, 24 and 28 respectively, readings.

State 4

State 4 also uses feedback control to modulate the flow of cold water tothe cooling coil 32 via valve 46, thereby controlling the amount ofenergy extracted from the air. However, in this case, the outdoor airdamper 12 is set at the minimum outdoor air position. Economizer logicis used to determine the transition to State 3. In the state diagramshown in FIG. 3, transition to State 3 occurs when the outdoor airtemperature is less than the changeover temperature minus the deadbandtemperature.

One of the main benefits of the present invention is that each ofcontrollers 302-306 can be independently tuned for each control loop.Another advantage is that it could be used with a distributed controlsystem. The current trend is towards using intelligent sensors andactuators that contain microprocessors.

For example, as shown in FIG. 6 a variable-air-volume (VAV) unit 600 isdesigned to maintain a constant supply air temperature setpoint anddeliver a variable amount of air into a controlled area of a building tomaintain the area at a desired temperature. VAV 600 is preferably anelectro-mechanical device with a digital controller 620. It is shown fora single duct application with a series fan arrangement. It will beappreciated that the invention has application to other implementationsof VAV unit 600. Having distributed digital control, VAV 600 may operatein a standalone manner, or may be coupled to a global control systemthrough a network arrangement such as the Metasys Network systemavailable from Johnson Controls, Inc. of Milwaukee, Wis. When coupled toa network, VAV 600 communicates using standard objects which residewithin the VAV controller 620. In this manner, controller 620 may retain"point" information which may be retrieve and viewed by a user at anyuser interface on the network. Preferably VAV 600 communicates throughtwo ports 642 and 644, one for an N2 bus connection and one for an N3bus, respectively. Moreover, when coupled to the network, VAV 600 isconfigurable through global tools with applications developed based uponstandard objects, assembly objects and nested applications such providedin the Metasys Application Basic Programming Language available fromJohnson Controls, Inc. Once the application is created using the globaltools, it may be downloaded to VAV 600 using an appropriate protocolsuch as BACnet.

With reference to FIG. 6, VAV 600 series fan 612 is, as observed,installed in series with the supply air stream. Variable air volume coldair is supplied into the fan chamber 618 from inlet duct 602 which iscoupled to the air distribution system for the building for receivingsupply air at the supply air temperature. Air flow sensor 604 providessupply air flow information, and damper 606 and damper actuator 608 actas a throttle for controlling the flow of supply air into VAV 600. Forexample, if cooling is not required, damper 606 may be adjusted to itsminimum flow setting. Additional air is drawn from the plenum duct 610which is coupled to receive air from the zone to maintain a relativelyconstant flow of air into the zone. Fan flow sensor 612 provides zoneair flow information, and flow adjust damper 614 allows for control ofthe zone flow. Finally, should heat be required, heat coil box 616 maybe activated to warm the air in fan chamber 616 and supplemental heatcoil 617 may be activated for warming air as it flows into the zone.

With continued reference to FIG. 6, VAV controller 620 is implemented inaccordance with the present invention as described above and includes aplurality of control elements 622-628 associated with each of the systemelements. VAV controller 620 includes box heating controller 622,supplemental heating controller 624, cooling controller 626 and flowcontroller 628. Though each of these controllers is shown associatedwith controller 620, it should be understood that each may beimplemented in a distributed fashion. That is, the specific controllermay be associated with the system element itself. For example, boxheating controller 622 may be implemented with box heating coil 616without departing from the fair scope of the invention. As described,these control elements may be implemented as discrete controllers or maybe implemented as a single controller acting on a number of sets ofcontrol parameters with a set being associated with each of the systemelements. In addition, VAV controller 620 contains the transition data630, preferably retained in memory within VAV controller 620,implementing the state transition logic illustrated in FIGS. 7a and 7bthereby determining the mode of operation based upon the current mode ofoperation, system performance data (i.e., flow rates and zonetemperatures) and setpoints all shown generally as inputs 632.

Control elements 622-628 may implement any suitable control strategy,and preferably, implement a control strategy optimized to the systemelement being controlled. In the preferred embodiment, box heatcontroller 622, supplemental heat controller 624 and cooling controllereach implement a proportional-integral-derivative feedback controllerusing the predictive adaptive control technology disclosed in theaforementioned commonly assigned United States Patents and United StatesPatent Application.

Referring then to FIGS. 7a and 7b, illustrated is a nested controlarchitecture in accordance with the preferred embodiments of the presentinvention. FIG. 7a illustrates the available modes of system operationand the transitions therebetween. FIG. 7b illustrates the operatingstates associated with the automatic operation mode and the transitionstherebetween. A mode is selected by the system user, using a networkinterface or interfacing directly with controller 620, setting the "HVACMode Request" equal to the selected mode. This is illustrated in FIG. 7aas a link, generally shown as 714, leading into a mode having the HVACMode Request set equal to the particular mode and a link, generallyshown as 716, exiting a mode having the HVAC Mode Request set not equalto the particular mode. The following is a description of the possiblemodes:

In Auto mode 700, the controller runs automatically to meet the controlobjectives. Within Auto mode are multiple states. The user requests Automode for the VAV controller 620 and the process dynamics determine thecurrent Auto state. For example, the current Auto state is Auto NoAction Required when the zone temperature is greater than the heatingzone temperature setpoint and the zone temperature is less than thecooling zone temperature setpoint. Each Auto state and the transitionsbetween these states are shown in FIG. 7b.

Two shutdown options will be available: shutdown box open 702 andshutdown box closed 704. The damper actuator at the shutdown box openmode 702 will control flow rate to satisfy the occupied cooling maximumflow rate setpoint. At the shutdown box closed mode 704, the damperactuator will be driven to the full closed end stop. When eithershutdown is enabled, all the devices that are associated with theoutputs of the controller (fan, supplemental heat, box heat, andlighting) are turned off.

As a window is opened, a binary input signifies the open state of thewindow, the HVAC Mode Control is forced to the window mode 706. Thewindow open status has the highest priority. Window mode 706 is similarto shutdown closed except it is dependent on a low limit temperaturesetpoint that will allow supplemental heat to operate. Upon therecognition of this mode, the damper is closed, fans are switched offand the supplemental heat is controlled to maintain the low limittemperature setpoint. In window mode 706, only the room temperature ismonitored, the box heating is disabled, and supplemental heating isenabled if the temperature falls below the low limit setpoint. The HVACMode Control can also be forced to window mode 706 by a supervisoryrequested command.

In warmup mode 708, known as the central system warmup, the air handlingunit provides warm air through the supply duct as needed to bring thesystem to normal occupied operating conditions. In warmup, the VAV flowsetpoint action is reversed. During warmup, supplemental heat is alwaysenabled, and the box heat and parallel fans are disabled by default.

Water flush mode 710 is typically used during the startup andcommissioning of VAV controllers on a new jobsite for the flushing,balancing, or maintenance of building heating water systems.Incremental, proportional, and two position-normally open and normallyclosed-heating outputs are affected by this feature.

Autocalibration mode 712 will be performed every operator defined periodwithin an application object. A counter, when expired, is used toinitiate autocalibration. Autocalibration will turn the parallel orseries box fan (if present) off. Autocalibration would drive the damperactuator closed for auto zero time, and calibrate the analog input flowsensor value based on zero flow. The autocalibration for each controllerwill be sequentially staggered so several controllers will notautocalibrate at the same time.

With reference now to FIG. 7b, the operating states of VAV unit 600 fromthe Auto mode 700 are shown. From the Auto mode 700 where no action isrequired, and with the HVAC Mode Request set equal to auto, VAVcontroller 620 will function without user intervention to meet thesystem performance criteria (user set points, energy consumptiondemands, flow requirements, etc.). A transition from a state, such as noaction 700 to auto cooling 720, occurs when the transition conditionsdefined by the transition data and illustrated on the links joining thestates, for example link 722, are met. Once a state is entered, forexample once the system has entered auto cool 720, it will remain inthat state until the transition conditions for exiting the state, forexample as shown on link 724, are met. The operating states of VAV 600include, in addition to no action 700 and auto cool 720, auto box heat726, auto supplemental heating with full box heating 728, autosupplemental heating 730 and auto box heating with full supplementalheating 732. One can see that the operation of the system can be easilyreconfigured. For example, should the user prefer box heating prior tosupplemental heating by defining transition data 630 appropriately boxheating 726 can be entered before supplemental heating 730, see forexample link 734. The concept of the state diagram also assists the userin viewing operation of the system.

With reference to FIGS. 8-11, a series of user interface displays areshown. In the preferred embodiment, the controller is part of networkedenvironmental control system including user interfaces having screendisplay. The user, by selecting the appropriate option, can retrieve thedisplay shown in FIG. 8 showing current system mode of operation.Referring then to FIG. 8, the main portion of the window is a statetransition diagram 800. State transition diagram 800 is representativeof a system including mechanical as well as ice storage cooling sources.There are 8 basic modes of operation (or states) 802-816 that arerepresented as circles. Transitions between states are represented bythe arrows, generally 815, connecting the circles 802-816. In each stateunique actions are being taken to control the system as has beendescribed. For example, FIG. 8 shows the controller in the DemandLimiting state 802. In this mode the controller uses both the chillerand ice storage to stay below a preset electric demand limit withoutdepleting the ice storage prematurely. As the day progresses the variousconditions indicated in field 818 may change, the status of theconditions being indicated by the check boxes 820. This in turn maycause the mode of operation to change. A change of state is indicated inthe window by a thick arrow 822 going from the previous mode ofoperation to the current mode of operation. For example, FIG. 8illustrates that when it was predicted that energy usage was"Approaching Demand Target", the corresponding box became checked, andthe controller switched from the Chiller Priority mode 806 to the DemandLimiting mode 802.

A description 830 of the current mode of operation and why this mode waschosen 832 can be obtained by using an input device and selectinganywhere in the state transition diagram screen 800. Upon making theselection, a State Transition Description window, FIG. 9, displays therequested information.

One can view the previous modes of operation by moving the scroll bar824 at the top right of the window. The caption underneath the scrollbar indicates what state (current or past) is displayed, for example,FIG. 10. The date and time information 826 at the bottom right of thewindow tells when the transition between the modes occurred. Field 818with the check boxes 820 illustrates the conditions of the variouscritical flags that determine the mode of operation. The bar graph 828to the right of the screen can show the status of a system element, forexample a percent utilization, and in this example, shows the iceinventory at the time the change occurred. Likewise, selecting anywherein this screen will retrieve a State Transition Description window, asshown in FIG. 9, for this previous state.

A Sys Var button 834 is provided and allows the user to retrieve andview the system variables, FIG. 11. If the current state is beingdisplayed, the current system variables are displayed. Selecting theplot button 836, provides a plot of the system variables versus time orby transition as requested by the user. If a past state is beingdisplayed, selecting the Sys Var button 834 displays the systemvariables values at that transition.

The present invention has been described in terms of preferredembodiments. Its many advantages, features and applications will bereadily appreciated by one of ordinary skill in the art. Its fair scopeis set forth the attached claims.

We claim:
 1. A system for controlling an environmental unit, whichincludes a heating element, a cooling element, and a ventilationelement; the system comprising:a sensor which produces a first signalindicating an environmental characteristic;a state machine controllercoupled to the sensor, the heating element, the cooling element, and theventilation element, the state machine controller having a plurality ofstates in which control signals are sent to different ones of theheating element, the cooling element, and the ventilation element; amemory coupled to the state machine controller and having a plurality ofdata structures, each of the plurality of data structures associatedwith one of the plurality of states and containing controller parametersdefining operation of the state machine controller in that one of theplurality of states and containing transition data defining at least onecondition that must exist for the controller to enter that one of theplurality of states; and a display device coupled to the state machinecontroller and to the memory, the display device operable forrepresenting each of the heating element, the cooling element and theventilation element with a respective display symbol and representingeach of the first set of transition data and the second set oftransition data as a display link joining respective ones of the displaysymbols.
 2. The environmental control system of claim 1 wherein thedisplay device comprises an input device which permits a user to selecta particular display link and wherein the display device is operable todisplay transition data associated with the particular display linkselected by the user.
 3. A system for controlling an environmental unit,which includes a heating element, a cooling element and a ventilationdamper; the system comprising:a temperature sensor which produces afirst signal indicating a sensed temperature; a state machine controllercoupled to the temperature sensor, the heating element, the coolingelement and the ventilation damper, the state machine controller havinga first state in which an activation signal is sent to the heatingelement, a second state in which an activation signal is sent to thecooling element, and a third state in which an activation signal is sentonly to the ventilation damper; a memory coupled to the state machinecontroller and having a first data structure containing a first set oftransition data specifying a first condition that must exist for thecontroller to enter the first state, a second set of transition dataspecifying a second condition that must exist for the controller toenter the second state, and a third set of transition data specifying athird condition that must exist for the controller to enter the thirdstate; and a display device coupled to the state machine controller andto the memory, the display device operable for representing each of theheating element, the cooling element and the ventilation damper with arespective display symbol and representing each of the first set oftransition data, the second set of transition data, and the third set oftransition data as display links joining respective ones of the displaysymbols.
 4. The environmental control system of claim 3 wherein thedisplay device comprises an input device which permits a user to selecta particular display link and wherein the display device is operable todisplay transition data associated with the particular display linkselected by the user.
 5. The environmental control system of claim 3wherein the memory further has a second data structure containing afirst set of control data for activating the heating element, a secondset of control data for activating the cooling element, and a third setof control data for activating the ventilation damper; and wherein thedisplay device comprises an input device which permits a user to selecta display symbol thereby causing the display device to display therespective set of control data.
 6. The environmental control system ofclaim 3 wherein the memory further has a second data structurecontaining a first set of control data for activating the heatingelement, a second set of control data for activating the coolingelement, and a third set of control data for activating the ventilationdamper.
 7. The environmental control system of claim 6 wherein the statemachine in the first state forms the activation signal in response tothe first set of control data, in the second state forms the activationsignal in response to the second set of control data, and in the thirdstate forms the activation signal in response to the third set ofcontrol data.
 8. A system for controlling an environmental unit, whichincludes a heating element, a cooling element and a ventilation damper;the system comprising:a temperature sensor which produces a first signalindicating a sensed temperature; a state machine controller coupled tothe temperature sensor, the heating element, the cooling element and theventilation damper, the state machine controller having a first state inwhich an activation signal is sent to the heating element, a secondstate in which an activation signal is sent to control a position of theventilation damper between a minimum open position and a maximum openposition, a third state in which an activation signal is sent to thecooling element and another activation signal is sent to open theventilation damper to greater than the minimum open position, and afourth state in which an activation signal is sent to the coolingelement and another activation signal is sent to place the ventilationdamper at the minimum open position; and a memory coupled to the statemachine controller and having a first data structure containing a firstset of transition data specifying a first condition that must exist forthe controller to enter the first state, a second set of transition dataspecifying a second condition that must exist for the controller toenter the second state, a third set of transition data specifying athird condition that must exist for the controller to enter the thirdstate, and a fourth set of transition data specifying a fourth conditionthat must exist for the controller to enter the fourth state.
 9. Thesystem as recited in claim 8 further comprising a display device coupledto the state machine controller and to the memory, the display deviceoperable for representing each of the heating element, the coolingelement and the ventilation damper with a respective display symbol andrepresenting each of the first set of transition data, the second set oftransition data, the third set of transition data, and fourth set oftransition data as display links joining respective ones of the displaysymbols.
 10. The environmental control system of claim 9 wherein thedisplay device comprises an input device which permits a user to selecta particular display link and wherein the display device is operable todisplay transition data associated with the particular display linkselected by the user.
 11. The environmental control system of claim 8wherein the memory further has a second data structure containing afirst set of control data for activating the heating element, a secondset of control data for activating the cooling element, and a third setof control data for activating the ventilation damper.
 12. Theenvironmental control system of claim 11 further comprising an inputdevice which permits a user to select a display symbol; and a displaydevice to display a set of control data associated with the displaysymbol selected via the input device.
 13. The environmental controlsystem of claim 11 wherein the state machine in the first state formsthe activation signal in response to the first set of control data, inthe second state forms the activation signal in response to the secondset of control data, in the third state forms activation signals inresponse to the second set of control data and the third set of controldata, and in the fourth state forms activation signals in response tothe second set of control data and the third set of control data. 14.The system as recited in claim 8 wherein the state machine controller inthe third state maintains the ventilation damper in substantially themaximum open position.